1. Field of the Invention
The present invention relates to a signal detecting device, a signal detecting method and a signal reproducing apparatus and, more particularly, to a detection device and method for detecting a pilot signal component from a modulated digital signal which contains information data and the pilot signal component, as well as to a signal reproducing apparatus arranged to perform tracking control by using such detection method.
2. Description of the Related Art
In recent years, with the advance of digital magnetic recording/reproduction technology, intensive development of digital VTRs have been carried out. In the field of the digital VTRs, various tracking control methods have been proposed because it is still necessary to perform tracking control during reproduction in the digital VTRs similarly to conventional analog VTRs.
One noticeable method includes the step of multiplexing a predetermined pilot signal component before a digital signal data stream to be recorded is digitally modulated and performing tracking control during reproduction by using the predetermined pilot signal component.
FIG. 1 is a block diagram schematically showing the construction of a recording system for a digital VTR which uses this kind of tracking control method. As shown in FIG. 1, rotary heads ch1 and ch2 are fixed to a rotary drum (not shown) with a phase difference of 180.degree. so that a modulated digital signal can be recorded on a magnetic tape T which is a recording medium.
The operation of the recording system will be described below.
A video signal which has been inputted from a terminal 1 is supplied to a digital recording-signal processing circuit 2. The digital recording-signal processing circuit 2 encodes the video signal into highly efficiently coded data and subjects the data to error-correcting encoding, thereby forming a digital data stream containing audio data and other auxiliary data in accordance with a recording data format.
This digital data stream is supplied to a digital modulation.pilot signal addition circuit 3. The digital modulation.pilot signal addition circuit 3 subjects digital modulation for adding redundancy to data, such as 24-25 conversion, to the data stream supplied from the digital recording-signal processing circuit 2, and adds pilot signal components to the modulated digital data stream by using the redundancy.
Specifically, for example, a leading bit "1" or "0" is added to the leading part of each 24-bit data to form two kinds of 25-bit data, and these two kinds of 25-bit data are outputted as bit streams, respectively. Then, these bit streams are individually subjected to NRZI modulation. Then, a DC component, a component f1 (the frequency of a first pilot signal) and a component f2 (the frequency of a second pilot signal) are extracted from each of the two kinds of NRZI-modulated bit streams, and the sum of these components is calculated. The sum is added to the accumulated value of the past sums, whereby accumulated values are formed for the respective bit streams. Then, these two kinds of accumulated values are compared with each other, and a bit stream of 25 bits which yields the smaller accumulated value is selected and outputted. The bit stream outputted in this case is a bit stream in which the DC component and the components fl and f2 are individually suppressed.
If a predetermined pattern signal is multiplexed with (for example, subtracted from) the extracted DC component, the aforesaid bit stream will have a frequency component relative to the predetermined pattern signal. Therefore, if the frequency of the predetermined pattern signal is set to the desired one of the frequency f1 and f2, a pilot signal component of desired frequency can be multiplexed with the modulated digital bit stream.
For example, if the bit rate of each bit stream is defined as fb; the frequency f1 of the first pilot signal as fb/90; and the frequency f2 of the second pilot signal as fb/120, a pattern signal which repeats its pattern at intervals of a 90- or 120-bit period can be subtracted from each of the two kinds of detected DC components. Thus, the pilot signal component of the desired frequency f1 or f2 can be multiplexed with the modulated digital bit stream.
The thus-obtained modulated digital bit streams are supplied to a switching circuit 4 as modulated digital recording signals, and are alternately supplied to the rotary heads ch1 and ch2 by head switching pulses (HSW) formed in accordance with the rotational phases of the respective rotary heads ch1 and ch2. The rotary heads ch1 and ch2 rotate with a phase difference of 180.degree. to alternately trace the magnetic tape T, thereby recording the modulated digital recording signals while sequentially forming a multiplicity of parallel helical tracks.
FIG. 2 shows one example of a recording pattern formed on the magnetic tape T. As shown, the first and second pilot signals are multiplexed on every other track from among the multiplicity of helical tracks formed in the above-described manner, whereby the first pilot signal of frequency f1 and the second pilot signal of frequency f2 are respectively multiplexed at intervals of a 4-track period in such a manner that the first and second pilot signals are alternately recorded on every other track. This recording pattern is formed, for example, by alternately subtracting the pattern signals having the frequency components f1 and f2 from the aforesaid DC component during every period in which the magnetic head ch1 makes a recording, and inhibiting either of the pattern signals from being subtracted from the DC component during every period in which the magnetic head ch2 makes a recording.
FIG. 3 is a block diagram showing one example of the construction of a conventional reproducing system for reproducing from the magnetic tape T the modulated digital recording signals recorded in the recording format shown in FIG. 2.
The modulated signals which have been alternately reproduced by the magnetic heads ch1 and ch2 are inputted to a head switching circuit 9 through reproducing amplifiers 7 and 8, respectively. The head switching circuit 9 is switched by the head switching pulse HSW supplied from a drum-rotation detecting circuit 15, whereby the modulated signals which have been alternately supplied from the magnetic heads ch1 and ch2 are formed into a continuous reproduced signal. This continuous reproduced signal is inputted to a digital reproduced-signal processing circuit 10, an f1 detecting circuit 12 and an f2 detecting circuit 13. The digital reproduced-signal processing circuit 10 performs predetermined kinds of precessing such as digital demodulation processing, error-correcting processing and decoding of highly efficiently coded data, and outputs the restored information data (video data) to an output terminal 11.
Each of the f1 detecting circuit 12 and the f2 detecting circuit 13 is formed by an analog band-pass filter and the like. The pilot signal components respectively extracted by the f1 and f2 detecting circuits 12 and 13 are supplied to a tracking control circuit 16. The tracking control circuit 16 detects the levels of the pilot signal components outputted from the respective circuits 12 and 13 and finds a difference between the detected levels. If tracking control is performed so that the magnetic head ch2 can reproduce the recorded tracks formed by the magnetic head ch2 itself, the magnetic head ch2 traces the tracks on which no pilot signals are multiplexed. Each time the magnetic head ch2 traces one of such tracks, the components f1 and f2 are obtained from both adjacent tracks. At this time, by finding a difference between the pilot signal components f1 and f2, it is possible to obtain a signal indicative of tracking error as to the magnetic head ch2. Incidentally, while the magnetic head ch1 is tracing, since no tracking-error signal is obtained, the tracking control circuit 16 samples and holds the immediately previous tracking-error signal. Also, since the polarity of the tracking-error signal reverses at intervals of a 2-track period, the aforesaid difference value is correspondingly reversed in polarity in synchronism with the head switching pulse HSW.
The thus-obtained tracking control signal is supplied to a capstan control circuit 17, and the capstan control circuit 17 controls the transport of the magnetic tape T so that each of the magnetic heads ch1 and ch2 can trace a desired track.
However, this conventional kind of apparatus has a number of disadvantages. One disadvantage is that analog band-pass filters are used to detect the respective pilot signals. External analog circuits must be especially provided in a digital VTR which is substantially made up of digital processing circuits, and a large space for installation of constituent components is needed in the digital VTR. Another disadvantage is that since higher harmonic components are produced by detecting circuits provided at the rear stages of the respective band-pass filters, low-pass filters or the like must be provided for eliminating these higher harmonic components. This disadvantage makes it difficult to achieve an efficient circuit arrangement.